The optimum mercury vapor pressure for production of 2537 .ANG. radiation to excite a phosphor coating in a fluorescent lamp is approximately six millitorr, corresponding to a mercury reservoir temperature of approximately 40.degree. C. Conventional tubular fluorescent lamps operate at a power density (i.e., typically measured as power input per phosphor area) and in a fixture configured to ensure operation of the lamp at or about a mercury vapor pressure of six millitorr (typically in a range from approximately four to seven millitorr); that is, the lamp and fixture are designed such that the coolest location (i.e., cold spot) of the fluorescent lamp is approximately 40.degree. C. Compact fluorescent lamps, however, including electrodeless solenoidal electric field (SEF) fluorescent discharge lamps, operate at higher power densities with the cold spot temperature typically exceeding 50.degree. C. As a result, the mercury vapor pressure is higher than the optimum four to seven millitorr range, and the luminous output of the lamp is decreased.
One approach to controlling the mercury vapor pressure in an SEF lamp is to use an alloy capable of absorbing mercury from its gaseous phase in varying amounts, depending upon temperature conditions. Alloys capable of forming amalgams with mercury have been found to be particularly useful. The mercury vapor pressure of such an amalgam at a given temperature is lower than the mercury vapor pressure of pure liquid mercury.
Unfortunately, accurate placement and retention of an amalgam to achieve a mercury vapor pressure in the optimum range in an SEF lamp are difficult. For stable long-term operation, the amalgam should be placed and retained in a relatively cool location with minimal temperature variation. Such optimal locations are at or near the tip, or apex, of the lamp envelope, or crown. Accordingly, it is desirable to place the amalgam in an optimal position near the cold spot of the lamp. Moreover, to achieve the desired beneficial effects of an amalgam in an SEF lamp, the amalgam should maintain its composition and optimized location during lamp processing and manufacturing steps as well as during lamp operation.